A mass spectrometer is an instrument which produces ions from a sample, separates the ions according to their mass-to-charge ratios by utilizing electric and magnetic fields, and provides output signals which are measures of the relative abundance of each ionic species present. The output signals are typically represented graphically such that the ion mass-to-charge ratios are shown on the x-axis, and the relative ion abundances are depicted on the y-axis to form a mass spectrum for the sample. The knowledge of the mass-to-charge ratios of the ions and the measured ion abundances allows a determination of the chemical composition of the sample molecules and their relative abundance.
It is desirable to calibrate the instrument to produce results with high mass-to-charge measurement accuracy and precision. This typically involves the introduction of a calibrant compound that produces ions of known mass-to-charge ratios in order to relate the measured mass-to-charge ratio to the known value for the calibrant compound. Current practice requires that for highly accurate measurements, the calibrant compound must be present at the same time as a measurement is to be made on a sample. This is often undesirable, since the calibrant peaks may interfere with the sample peaks to be measured. It is preferable to perform the calibration separately, and then make the sample measurement at a later time. This may be referred to as "external calibration," since the calibrant compound is not present in the mass spectrometer at the time that the sample is measured. This has been difficult or impossible to accomplish with conventional mass spectrometers (magnetic sector instruments) due to fluctuations and instabilities in the electric and magnetic fields employed by the mass analyzers.
In the calibration of ion cyclotron resonance (ICR) mass spectrometers, the measured frequency, f, can be related to the mass, m, of a given ion by the relation, EQU m=k.sub.1 B/f+k.sub.2 E/f.sup.2
where k.sub.1 and k.sub.2 are constants, B represents the magnetic field, and E represents the electric field experienced by the ions. See E. B. Ledford, Jr. et al., "Space Charge Effects in Fourier Transform Mass Spectrometry. Mass Calibration," Anal. Chem., vol. 56, no. 14, 1984, pp. 2744-2748. If a superconducting magnet is employed, the magnetic field, B, is stable for long periods of time, and may be considered to be constant for all practical purposes. The electric field term is related to the ion-trapping cell geometry (i.e. the arrangement of electrodes used for confinement detection of ions), the potentials applied to the trapping plates (i.e. the electrodes placed perpendicular to the magnetic field), and the number of ions present in the ion trapping cell.
Since the cell geometry is fixed, and the potentials applied to the trapping plates may be controlled by the operator, the major source of instability in an external calibration results from changes in the number of ions from the time when the calibration is performed, to the time when the sample measurement is made. Methods for estimating the number of ions present in the cell have been proposed that are based on the total gas pressure and characteristics of the electron beam (current, path length, etc.). See R. L. White, et al., "Exact Mass Measurement in the Absence of Calibrant by Fourier Transform Mass Spectrometry," Anal. Chem., vol. 55, no. 2, 1983, pp. 339-343. External calibration was demonstrated for cases where the calibration and sample measurement are made under conditions which produce approximately similar numbers of ions. See C. L. Johlman et al., "Accurate Mass Measurement in the Absence of Calibrant for Capillary Column Gas Chromatography/Fourier Transform Mass Spectrometry," Anal. Chem., vol. 57, no. May 6, 1985, pp. 1040-1044. While these methods have enjoyed some success, they are not completely satisfactory. The former approach is dependent upon an estimate of the number of ions produced by a single ionization method (electron ionization), which may be subject to error. The second method relies upon the assumption that the number of sample ions is roughly the same as the number of calibrant ions. Often, this is not the case.
Another method for external calibration is based upon measurement of the frequency of the first upper sideband of the resonant frequency of the ion to be measured. See M. Allemann et al., "Sidebands in the ICR Spectrum and their Application for Exact Mass Determination," Chem. Phys. Lett., vol. 84, no. 3, Dec. 15, 1981, pp. 547-551, and U.S. Pat. No. 4,500,782 entitled "Method of Calibrating Ion Cyclotron Resonance Spectrometers" and issued to Allemann et al. The frequency of this upper magnetron sideband is approximately equal to the true cyclotron frequency of the ion to be measured, and is not affected by changes in the trapping voltage. The magnetron sidebands to be measured are much smaller in intensity than the main peak, and usually require high resolution to separate them from the main sample peak. Alternatively, several calibrant masses may be measured, and the difference between the measured mass and the calculated cyclotron frequency is used as a correction factor to convert the measured frequency to the cyclotron frequency for an unknown ion.
Sidebands may also result from the coupling of cyclotron motion and trapping motion. See, e.g., B. S. Freiser, et al., "Observation of Ion Ejection Phenomena in Ion Cyclotron Resonance Experiments," Int. J. Mass Spec. Ion Physics., vol. 12, 1973, pp. 249-255; K. Aoyagi, "Study of the Quasi-peaks in the Ion Cyclotron Double Resonance," Bull. Chem. Soc. Japan, vol. 51, 1978, pp. 355-359.
Excitation of the axis (trapping) motion of ions at frequencies which correspond to the frequencies for trapping sidebands has been observed. See W. J. van der Hart, et al., "Excitation of the Z-Motion of Ions in a Cubic ICR Cell," Int. J. Mass Spec. Ion Proc., vol. 82, 1988, pp. 17-31.